In the medical area, a number of degradable polymers have been developed that break down in vivo into their respective monomers within weeks or a few months. Despite the availability of these synthetic degradable polymers, there is still a need to develop degradable polymers which can further extend the range of available properties, particularly mechanical properties.
Polyhydroxyalkanoates are natural, thermoplastic polyesters and can be processed by traditional polymer techniques for use in an enormous variety of applications, including consumer packaging, disposable diaper linings and garbage bags, food and medical products. Initial efforts focused on molding applications, in particular for consumer packaging items such as bottles, cosmetic containers, pens, and golf tees. U.S. Pat. Nos. 4,826,493 and 4,880,592 describe the manufacture of poly-(R)-3-hydroxybutyrate (“PHB”) and poly-(R)-3-hydroxybutyrate-co-(R)-3-hydroxyvalerate (“PHBV”) films and their use as diaper backsheet. U.S. Pat. No. 5,292,860 describes the manufacture of the PHA copolymer poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) and the use of these polymers for making diaper backsheet film and other disposable items. Diaper back sheet materials and other materials for manufacturing biodegradable or compostable personal hygiene articles from PHB copolymers other than PHBV are described in PCT WO 95/20614, WO 95/20621, WO 95/23250, WO 95/20615, WO 95/33874, WO 96/08535, and U.S. Pat. Nos. 5,502,116; 5,536,564; and 5,489,470.
One of the most useful properties of PHAs which readily distinguishes them from petrochemically-derived polymers is their biodegradability. Produced naturally by soil bacteria, PHAs are degraded upon subsequent exposure to these same bacteria in either soil, compost, or marine sediment. Biodegradation of PHAs is dependent upon a number of factors, such as the microbial activity of the environment and the surface area of the item. Temperature, pH, molecular weight, and crystallinity also are important factors. Biodegradation starts when microorganisms begin growing on the surface of the plastic and secrete enzymes which break down the polymer into hydroxy acid monomeric units, which are then taken up by the microorganisms and used as carbon sources for growth. In aerobic environments, the polymers are degraded to carbon dioxide and water, while in anaerobic environments the degradation products are carbon dioxide and methane (Williams & Peoples, CHEMTECH, 26:38–44 (1996)). While the mechanism for degradation of PHAs in the environment is widely considered to be via enzymatic attack and can be relatively rapid, the mechanism of degradation in vivo is generally understood to involve simple hydrolytic attack on the polymers' ester linkages, which may or may not be protein mediated. Unlike polymers comprising 2-hydroxyacids such as polyglycolic acid and polylactic acid, polyhydroxyalkanoates normally are comprised of 3-hydroxyacids and, in certain cases, 4-, 5-, and 6-hydroxyacids. Ester linkages derived from these hydroxyacids are generally less susceptible to hydrolysis than ester linkages derived from 2-hydroxyacids.
Researchers have developed processes for the production of a great variety of PHAs, and around 100 different monomers have been incorporated into polymers under controlled fermentation conditions (Steinbüchel & Valentin, FEMS Microbiol. Lett., 128:219–28 (1995)). There are currently only two commercially available PHA compositions: PHB and PHBV. Because of their great compositional diversity, PHAs with a range of physical properties can be produced (Williams & Peoples, CHEMTECH, 26:38–44 (1996)). The commercially available PHAs, PHB and PHBV, represent only a small component of the property sets available in the PHAs. For example, the extension to break of PHB and PHBV range from around 4 to 42%, whereas the same property for poly-4-hydroxybutyrate (“P4HB”) is about 1000% (Saito & Doi, Int. J. Biol. Macromol. 16: 99–104 (1994)). Similarly, the values of Young's modulus and tensile strength for PHB and PHBV are 3.5 to 0.5 GPa and 40 to 16 MPa, respectively (for increasing HV content to 25 mol %), compared to 149 MPa and 104 MPa, respectively for P4HB (Saito & Doi, Int. J. Biol. Macromol. 16: 99–104 (1994)).
PHB and PHBV have been extensively studied for use in biomedical applications, in addition to their commercial use as a biodegradable replacement for synthetic commodity resins. These studies range from potential uses in controlled release (see, e.g., Koosha, et al., Crit. Rev. Ther. Drug Carrier Syst. 6:117–30 (1989) and Pouton & Akhtar, Adv. Drug Delivery Rev., 18:133–62 (1996)), to use in formulation of tablets, surgical sutures, wound dressings, lubricating powders, blood vessels, tissue scaffolds, surgical implants to join tubular body parts, bone fracture fixation plates, and other orthopedic uses, as described in PCT WO 98/51812. Wound dressings made from PHB are disclosed in GB 2166354 A to Webb, et al. One advanced medical development is the use of PHB and PHBV for preparing a porous, bioresorbable flexible sheet for tissue separation and stimulation of tissue regeneration in injured soft tissue described in EP 754467 A1 to Bowald et al. and EP 349505 A2. Reports have also described the use of PHBV to sustain cell growth (Rivard, et al., J. Appl. Biomat., 6:65–68 (1995)).
Besides biocompatibility, it is often desired that an implanted medical device should degrade after its primary function has been met. PHB and PHBV, the only PHAs tested as medical implants to date, have shown very long in vivo degradation periods, of greater than one year for PHB (Duvernoy, et al. Thorac. Cardiovasc. Surgeon 43:271–74 (1995); Malm, et al., J. Thorac. Cardiovasc. Surg. 104:600–07 (1992)). For many applications, this very long degradation time is undesirable as the persistence of polymer at a wound healing site may lead to a chronic inflammatory response in the patient. Slowly degrading PHB patches used to regenerate arterial tissue have been found to elicit a long term (greater than two years) macrophage response (Malm, et al., Eur. Surg. Res. 26:298–308 (1994)). Macrophages were identified as being involved in the degradation of the PHB implants and this long term macrophage response appears to indicate the presence of persistent, slowly degrading particulate material originating from the implant. Although a PHB patch used for repair of the pericardium was not seen by ordinary light microscopy after 12 months implantation, small residual particulate material was observed by polarized light microscopy (Malm, et al., Scand. J Thor. Cardiovasc. Surg. 26:9–14 (1992)). It is unclear whether this particulate material remains localized at the implant site or migrates throughout the body, possibly causing unforeseen complications. The biological fate, or medical impact of this particulate material, cannot be predicted without long term study. In order to minimize potential problems associated with slowly degrading PHAs, it is advantageous to utilize resorbable materials with faster in vivo degradation rates.
There has been only one report describing the biocompatibility or in vivo degradation of any other PHA polymer in biomedical applications (PCT WO 98/51812). U.S. Pat. No. 5,334,698 to Witholt et al. discloses medical articles manufactured with an optically active polyester isolated from Pseudomonas oleovorans cells; however, no examples or discussion of fabrication or biocompatibility testing are shown, and no methods are provided to obtain the polymer in a suitably pure form for in vivo medical use. Since bacteria suitable for production of these polymers may also produce an endotoxin, as well as other inflammatory mediators, it is important that the polymer be processed to remove these contaminants.
For many applications, the rate of PHA biodegradation is well suited to the required product lifetime. However, in certain cases it would be desirable to be able to exert more control over the rate at which the polymers breakdown in the environment. Such control would extend the range of applications for this class of polymers. For example, a PHA film may have suitable mechanical properties to be used as a mulch film, yet not have the most optimum rate of degradation for the application. The ability to be able to control the rate of degradation of the polymer in the environment would thus be a distinct advantage.
U.S. Pat. No. 5,935,506 discloses a PHB stent. The stent construct, which is reported to bioresorb rapidly, contains a large amount of plasticizer. However, the plasticized PHB approach fails to work, as greater than 90% stent stenosis was shown at four weeks (see Behrend, American J. Cardiol. p. 45, TCT Abstracts (October 1998); Unverdorben, et al., American J. Cardiol. p. 46, TCT Abstracts (October 1998)). It would be advantageous to provide a bioresorbable stent with improved mechanical properties and no plasticizer.
Thus while the polyhydroxyalkanoates offer a wide range of mechanical properties which are potentially useful in medical applications, their use particularly in vivo as resorbable polymers has been limited by their slow hydrolysis. It would thus be desirable to develop methods for controlling the rates of degradation of polyhydroxyalkanoates.
PCT WO 98/51812 discloses methods for making a wide range of biodegradable biocompatible polyester materials known as polyhydroxyalkanoates. These materials are made in high purity, and are suitable for use in in vivo medical applications.
It is therefore an object of this invention to provide new devices and uses for compositions comprising or derived from polyhydroxyalkanoates which degrade more readily in the environment and/or in vivo.
It is another object of this invention to provide methods for fabricating the articles and devices from these compositions.